This invention relates generally to a scanning system employing near field optics, and more particularly to a system which includes solid immersion lenses mounted on cantilevers and to methods of fabricating the cantilevers with integral solid immersion lens.
Near field optical scanning has in recent years been applied to optical imaging, magneto-optic (MO) storage and lithography. In near field optical scanning, light pixels of sub-wavelength dimensions are achieved by scanning an aperture of sub-wavelength dimensions or by scanning a solid immersion lens very close to a sample.
A photolithography system employing near field optics is described in U.S. Pat. No. 5,517,280 which is incorporated herein in its entirety by reference. The photolithography system described in the ""280 patent includes an array of cantilevers, preferably formed in a silicon wafer. Each cantilever includes a transparent pyramidical or conical tip located near the free end of the cantilever.
The array of cantilevers is positioned adjacent a wafer which is to be lithographed, in the manner of an atomic force microscope operating in the attractive mode. Each cantilever is a compound structure, including a thick portion and a thin portion, the latter having a preselected mechanical resonant frequency. The cantilevers are caused to vibrate at their resonant frequency and the actual frequency of vibration is detected and used to maintain a uniform spacing between the tip of the cantilever and the surface of a photoresist layer which is to be exposed. Both the vibrational motion and the control of the tip-photoresist spacing are effected by means of a capacitive plate located adjacent the cantilever.
The cantilever array is scanned over a photoresist layer on the wafer, preferably in a raster pattern, and the individual light switches are operated so as to expose individual pixels on the photoresist layer to pattern the photoresist.
Another approach for high resolution lithography, optical microscopy or MO storage is to use near field optical imaging techniques for forming the spot or pixel. In near field optical imaging techniques, a sharp tip (usually an optical fiber) is raster scanned across a surface and is used to collect or deliver light. With sharp tips, resolutions of 12 nm have been obtained. These fibers are typically made by stretching and heating an optical fiber to narrow it and then cleaving it. The tapered tip of the fiber is then coated with metal (to prevent light from leaking out the sides) except at the very end of the tip to form an aperture.
Tapered fibers do not propagate waves in the region where the diameter of the waveguide is less than approximately xcex/2n where n is the refractive index of the fiber, so in this tapered region the waveguide is cut off and the losses are exponential with distance. Hence, optical fiber probes with 100 nm apertures typically have transmission efficiencies of 10xe2x88x925 to 10xe2x88x926. Thus, tapered optical fiber probes typically have very poor efficiency, making them unsuitable for high speed lithography, scanned imaging, microscopy or optical storage. Furthermore, the probes are usually manufactured singly and are difficult to integrate into arrays, which prevents their use for parallel imaging, lithography or optical storage.
An enhancement of the near-field technique involves the addition of a solid immersion lens (SIL) to the optical system. The use of an SIL in a lithographic application has been discussed by T. R. Corle, G. S. Kino, and S. M. Mansfield in U.S. Pat. No. 5,121,256 (issued Jun. 9, 1992). The SIL in that arrangement has a hemispherical surface on one side of the lens to improve the coupling of light into the lens, and a flat surface on an opposing side of the lens facing the sample. Evanescent fields just outside the flat surface of the SIL improve the resolution by a factor of 1/nSIL, where nSIL is the index of refraction of the material from which the lens was made. For typical materials, nSIL=1 to 2.5, so that a microlithography system which was capable of 0.5 xcexcm resolution will be capable of 0.2 to 0.25 xcexcm resolution with an SIL.
Solid immersion lenses may be used in applications other than lithography. The addition of an SIL to a microscopic imaging system has been described by G. S. Kino and S. M. Mansfield in U.S. Pat. No. 5,004,307 (issued Apr. 2, 1991). The addition of an SIL to an optical recording system has been disclosed by T. R. Corle, G. S. Kino, and S. M. Mansfield in U.S. Pat. No. 5,125,750 (issued Jun. 30, 1992).
It is an object of the present invention to provide an optical scanning system which includes one or more cantilevers. Each cantilever includes a tip near its free end which comprises a solid immersion lens and a conical or pyramidical tip which may be sharp or flat at its end. The tip efficiently guides impinging electromagnetic energy to the aperture. The electromagnetic energy can be in the form of ultraviolet (UV), infrared or visible light. The light impinging on the tip is digitally controlled whereby to form pixels on an associated surface for lithography, storage or microscopy. In operation, the tip of the cantilever(s) is brought very close to the surface to be scanned and the tips and surface are moved relative to one another whereby to scan a plurality of electromagnetic pixels or spots on the surface. A feedback control controls the distance between the surface and the tip to assure that the tip is operated in the near field mode.
It is a general object of the present invention to provide a cantilever with a tip including a solid immersion lens with a conical or pyramidical tip and to a method of fabrication.
It is another object of the present invention to provide an array of tips with optical near field apertures integrated on cantilevers and to a method of fabrication.
It is a further object of the present invention to provide a method of fabricating extremely small lenses of a few microns diameter for use in multiple lens arrays.
It is another object of the present invention to provide an extremely small aspheric solid immersion lens and to a method of fabrication.
The foregoing and other objects of the invention are achieved by a method of micromachining a solid immersion lens using chemical and plasma etching techniques. The micromachining method is extended to provide a cantilever or array of cantilevers each with a solid immersion lens at the tip. The invention also includes incorporating an array of such cantilevers into an optical scanning system.